Study On Macro And Micro Mechanical Properties Of SiC_f/Ti-6Al-4V Composites

Due to its high specific strength, specific stiffness and specific modulus as wellas excellent high temperature properties and fatigue properties, continuous SiC fiberreinforced Ti alloy matrix composites (SiCf/Ti composites) can be used for theaviation mechanic parts including blade, casing and blisk to reduce weight ofaeroengine greatly and improve efficiency of aircraft. However, the successfulapplication of SiCf/Ti composites is determined by mechanical properties. One causeis that high residual stress is produced by difference of thermal expansion coefficientbetween fiber and matrix with cooling from preparation temperature to roomtemperature, and another is that SiCf/Ti composites showed anisotropism and itstransverse properties are far lower than longitudinal properties, even though thetransverse force is inevitable during service. In addition, SiCf/Ti composites oftenwork in high temperature condition, and a major performance for titanium alloymatrix is the creep. Therefore, an overall study on residual stresses, longitudinalproperties, transverse properties and creep properties plays an important guiding rolein improvement of preparation technology and mechanical properties for SiCf/Ticomposites.The effects of fiber packing pattern, fiber volume fraction and matrix propertieson radial residual stress, axial residual stress and hoop residual stress are analyzed bythree-dimensional model established thorugh ANSYS finite element software. Theresults show that the nonuniformity of radial residual stress distribution at interfacesalong fiber circumferential direction increases with increasing fiber volume fraction,while the increasing extent of nonuniformity is far smaller with hexagonal fiber arraythan that with aquare and square diagonal fiber array. The axial residual stress atinterfaces is well distributed along fiber circumferential direction regardless of fiberpacking pattern especially for high fiber volume fraction. The change of hoop residualstress is similar to that of radial residual stress. The closer thermal expansioncoefficient between fiber and matrix is, the lower thermal residual stress and the betterdistribution is along fiber circumferential direction. And the higher matrix modulus is,the higher interfacial residual stress for composites. The interfacial residual stress isnot changed with yield strength when yield strength of matrix is higher than a criticalvalue. On the contrary, the interfacial residual stress decreases with decreasing ofyield strength. The effect of interfacial shear strength, fiber volume fraction and early breakageof a few fiber on longitudinal tensile properties of SiCf/Ti-6Al-4V composites isinvestigated by the method of combining finite element with Monte-Carlo. The resultsshow that fracture surfaces of the fibers in composites are non-coplanar with weakbond interfaces and two “knee”(corresponding to interfacial debonding of brokenfiber and matrix yielding) appears in stress-strain curves due to early breakage of afew fiber, and longitudinal tensile strength of composites decreases. The catastrophicfailure of composites occurs, and the fibers breakage is nearly on the same plane withstrong bond interfaces, but the tensile strength of composites remains essentiallyconstant despite of early breakage of a few fiber. Fiber volume fraction has a greateffect on tensile strength of composites. As increase of fiber volume fraction, tensilestrength of composites increases, while corresponding strain is reverse.Interface and mechanical behaviors of SiCf/C/Ti-6Al-4V composites areobversed by Scanning Electron Micoscopy (SEM) and Energy DispersiveSpectrdmeter (EDS). The results show that interface layer thickness increases withincreasing exposure temperature and prolonging exposure time. Interfacial reactionforms only TiC with remaining C coating. But after C coating is consumed, besidesTiC, Ti Silicide including Ti3SiC2and Ti5Si3are formed by interfacial reaction. Manyfibers are pulled out from matrix during longitudinal tension for SiCf/C/Ti-6Al-4Vcomposites as-processed and heat-treated at750℃for36h. Fracture step in matrix isobsvered in the region with nonuniform and even lapping fiber distribution. However,SiCf/C/Ti-6Al-4V composites exhibit an obvious brittle fracture feature after exposureat900℃for60h during longitudinal tension, and fiber breakage is nearly on the sameplane without pulling out.The interfacial failure mechanisms and corresponding transverse properties arediscussed by two-dimensional plane strain model based on periodicity of fiberdistribution under transverse loading for SCS-6/Ti-6Al-4V composites. The resultsshow that the interfaces are inclined to radial failure with both high and low fibervolume fraction with high interfacial bond strength, and the higher temperature is, thelarger possibility of radial failure is. The interfaces incline that shear failure caused byradial failure or shear failure first and then radial failure while the composite is withlow interfacial bond strength, and the latter is more likely to occur with decrease oftemperature and fiber volume fraction. Under transverse displacement loading, thestress-strain curves of composites show only one “knee”with perfectly strong bondinterfaces and the curves contain two “knee” with perfectly weak bond interfaces.Zig-zag curves appear while the interface is radial failure. Three “knee” will be seen ifthe interface failes by shear firstly and then along radial. And they show thediscontinuity of strain when shear failure causes radial failure at interfaces. The effect of axial residual stress on maximum creep stress and strain as well asthe stress supported by fiber, matrix and interface layer during creep is derived byanalytical model. On the basis, the relationship between creep strain rate and time,and effect of interfacial characteristic on creep properties of composites is studied.